1. Field of the Invention
The present invention relates to an electric actuator comprising a driving force-transmitting belt for transmitting the rotary driving force of a rotary driving source to a slider so that the slider is moved.
2. Description of the Related Art
An electric actuator has been hitherto widely used as a means for transporting a workpiece or the like, in which a timing belt is driven by the rotary driving force of a rotary driving source such as a motor to displace a slider for transporting the workpiece.
A tension-adjusting means is provided for the timing belt of a resin material in order to adjust the tension when the timing belt is loosened during the assembling or due to the use for many years.
As the tension-adjusting means for the timing belt, for example, a pair of pulleys are used. The timing belt runs over the pulleys. The pulleys are displaced in directions to approach or separate from each other so that the tension of the timing belt is adjusted.
Further, for example, a pair of connecting members, which are connected to both ends of the timing belt, are installed to a slider. The tension of the timing belt is adjusted by making the pair of connecting members approach or separate from one another.
As shown in FIG. 25, a conventional electric actuator 1 comprises a guide rail 3 which is provided along the axis of a rectangular housing 2, and a slide block 4 which transports the workpiece by sliding along the guide rail 3.
In the housing 2, there are a driving pulley 5a which is rotated by an unillustrated rotary driving source, and a plurality of driven pulleys 5b which are arranged in the four corners of the housing 2. The driving pulley 5a is rotated by the unillustrated rotary driving source, and a timing belt 6 running over the driving pulley 5a and the driven pulleys 5b is circled or orbited by a predetermined distance. Both ends of the timing belt 6 are connected to belt attachment fixtures 7a, 7b. The belt attachment fixtures 7a, 7b are installed to the upper surface of the slide block 4 by attachment screws 8. The belt attachment fixtures 7a, 7b and the timing belt 6 are integrally connected to one another by screw members 9.
Elongate holes 11a, 11b extend in the axial direction through fastening sections 10a, 10b which are provided at upper portions of the belt attachment fixtures 7a, 7b. 
A groove 12 is formed on the upper surface of the slide block 4, and the belt attachment fixtures 7a, 7b are slidably engaged with the groove 12 in the axial direction. The groove 12 is recessed by a predetermined depth on the upper surface of the slide block 4.
When the tension is applied to the timing belt 6, the attachment screws 8 to fix the belt attachment fixtures 7a, 7b to the slide block 4 are loosened, and one belt attachment fixture 7a is displaced in the direction to approach the other belt attachment fixture 7b along the groove 12 of the slide block 4. In this case, the belt attachment fixture 7a, 7b is displaced in the groove 12 along the shape of the elongate hole 11a, because the attachment screws 8 are engaged with the elongate hole 11a which extends in the axial direction.
As a result, the ends of the timing belt 6 are pulled in the directions to approach one another. Therefore, the tension of the timing belt 6 is increased. When the attachment screws 8, which are temporarily tacked to the slide block 4, are tightened to completely fix the belt attachment fixtures 7a, 7b, the adjusted tension of the timing belt 6 is retained (see, for example, Japanese Laid-Open Patent Publication No. 9-89067).
In the case of the conventional electric actuator 1 shown in FIG. 25, if the driving pulley 5a or the driven pulley 5b, over which the timing belt 6 runs, is displaced to adjust the tension of the timing belt 6, it is necessary to secure the space for displacing the driving pulley 5a or the driven pulley 5b in the axial direction. Therefore, the size or dimension of the electric actuator 1 in the axial direction is increased, and the entire electric actuator 1 is large.
In relation to the method in which the tension of the timing belt 6 is adjusted by displacing the belt attachment fixtures 7a, 7b in the axial direction, the attachment positions of the fastening sections 10a, 10b of the belt attachment fixtures 7a, 7b to be attached to the slide block 4 are offset with respect to the attachment positions of the timing belt 6 in the X-Y plane as viewed in the direction of the arrow Z. Therefore, the moments are caused by the tension of the timing belt 6, and are exerted on the belt attachment fixtures 7a, 7b in the directions (directions of the arrows Q) to pull both ends of the timing belt 6 through the fastening sections 10a, 10b. As a result, it is difficult to correctly adjust the tension of the timing belt 6.
Another method is explained in FIG. 26, for example. In this case, end blocks 13a, 13b are arranged at both ends of an operation mechanism 19. A timing belt 16, which transmits the rotary driving force supplied from a driving mechanism 14 to an operating section 15, has its ends 16a, 16b which are fixed to the end blocks 13a, 13b by fixing pieces 17a, 17b, respectively.
The ends 16a, 16b of the timing belt 16 are inserted into attachment portions of the end blocks 13a, 13b while the fixing pieces 17a, 17b are engaged with teeth of the timing belt 16. When two screw members 18a, 18b, which are screwed with each of the fixing pieces 17a, 17b, are tightened, the ends 16a, 16b of the timing belt 16 are integrally fixed to the end blocks 13a, 13b (see, for example, Japanese Laid-Open Patent Publication No. 63-134191).
In the conventional operation mechanism 19 shown in FIG. 26, the ends 16a, 16b of the timing belt 16 are fixed to the end blocks 13a, 13b by screwing the two screw members 18a, 18b into each of the fixing pieces 17a, 17b and pressing the fixing pieces 17a, 17b toward the timing belt 16.
Therefore, when attachment operation is performed for the timing belt 16, the operation is complicated to fix the timing belt 16 with the fixing pieces 17a, 17b by screwing the plurality of screw members 18a, 18b. Further, the number of parts is increased, such as the plurality of fixing screw members 18a, 18b. 
When the timing belt 16 is fixed by tightening the screw members 18a, 18b, the tightening force may be changed depending on the respective operators, and the screw members 18a, 18b may be excessively tightened. Therefore, excessive pressing force may be exerted on the timing belt 16 by the fixing pieces 17a, 17b, and the durability of the timing belt 16 may be deteriorated.
On the other hand, if the tightening force is changed depending on the respective operators, and the tightening force to tighten the screw members 18a, 18b is decreased, then the timing belt 16 may be loosened. As a result, the attachment of the timing belt 16 by the fixing pieces 17a, 17b is unstable due to the difference of the tightening force depending on the respective operators.